Comparison of New Protein Systems for Lactating Dairy Cows

Abstract

Four protein systems from the United States and five protein systems from Europe are compared to National Research Council dairy cattle requirements for protein. Predicted minimum protein in dietary dry matter ranges from 9 to 13% at 10 kg milk/day and 11 to 17% at 40 kg milk/day. Predieted optimum undegradabilities range from 7 to 41% at 10 kg milk/day and 20 to 55% at40 kgmilk/day. Relative changes of concentration and undegradability of protein are affected largely by relative changes of digestibility and intake of energy, respectively. Protein flow into the small intestine as predicted from these systems was generally less than expected from three equations based on digestible organic matter intake alone or digestible organic matter intake and insoluble nitrogen intake. The percentage of intake nitrogen excreted as fecal nitrogen for three systems was considerably less than that expected from two equations based on concentration of dietary protein. The dietary nitrogen excreted in urine ranged from 26 to 54% at 10 kg milk/day and from 24 to 44% at 40 kg milk/day. The dietary nitrogen excreted in milk ranged from 24 to 34% at 10 kg milk/day and from 33 to 48% at 40 kg milk/day. The failure to include a fecal metabolic protein fraction gave low protein requirements, low optimal undegradabilities, and low excretion of dietary nitrogen in feces. A fecal metabolic protein fraction is needed to produce the hyperbolic fecal nitrogen curve. The fecal metabolic protein fraction is functionally related to intake of dry matter and should be a separate component of requirement. The energy components